A conventional rotation angle detection device detects a rotation angle of a rotary shaft of a motor, for example, by calculating a rotation angle based on an output signal produced from a sensor element. It is known to determine abnormality when abnormality arises in an output signal of a sensor element (for example, patent document 1).
Patent document 1: JP 2005-49097A (EP 1503184 A2)
According to the technology in patent document 1, sin θ and −sin θ, which are output signals of a bridge circuit, are applied to an adder to generate a signal A, from which sine components are subtracted. Similarly cos θ and −cos θ, which are also output signals of the bridge circuit, are applied to an adder to generate a signal B, from which cosine components are subtracted. Abnormality is determined, if the signal A or B exceeds an upper limit value or falls below a lower limit value. The abnormality is determined based on the signal A or B generated by the adder according to patent document 1. It is however not possible to specify which one of the output signals is abnormal. It is also not possible to detect the rotation angle if one of the cosine signal and the sine signal is lost, because only the other one of the signals remains available.